1. Field of the Invention
The present invention concerns a process for the production of polymer compositions. In addition, the present invention concerns films prepared of bimodal polymer compositions obtained by the present process. In particular, the present invention relates to bimodal films having an improved balance between the optical and mechanical properties and a good processability.
2. Description of Related Art
The processability on a film blowing line as well as the physical properties of the final film depend largely on the polymer structure, especially on the molecular weight distribution (MWD). If the polymer is bimodal, i.e., if the MWD is broad, the polymer can be expected to exhibit a good processability. Other important properties, which naturally depend on the application the polymer material is used in, comprise optical properties (i.e., the film should be clear and glossy) and mechanical properties.
Conventionally, linear low density polyethylene (PE-LLD) having a bimodal molecular weight distribution is produced by polymerization in the presence of Ziegler catalysts in two cascaded reactors. Similarly, high density polyethylene (PE-HD) having a bimodal MWD has been produced by polymerization in the presence of Ziegler catalysts in two reactors in series.
Medium density polyethylene (PE-MD) for blown film is typically supplied by unimodal Cr-based products. These materials are extensively used in coextruded films as a stiffness-improvement, but give relatively less contribution to other physical properties like impact required by packaging.
The use of a metallocene catalyst in a two-stage polymerization process is known from EP-A-447035 and EP-A-881237. Bimodal polyethylene for film is known from e.g. EP-A-605952, EP-A-691353. EP-A-691367 and WO-A-9618662.
EP-A-447035 discloses an ethylene polymer composition having a density of 860-940 kg/m3 and an intrinsic viscosity of 1-6 dl/g, which would mean with a rough calculation a MFR2 in a range of approximately 0,04-60 g/10 min. The composition has been produced using a catalyst comprising a ligand that has a cycloalkadienyl skeleton and an organoaluminum oxy-compound. The publication refers to the reduced fraction of polymer soluble in n-decane in the resins produced according to the invention. It further states that when the fraction of such polymer is low the polymer has excellent anti-blocking properties.
In addition, the publication states that the target has been to produce materials having the clarity of the unimodal metallocene-based resin but a superior processability. The publication does not, however, disclose whether or not the good clarity and improved processability actually were achieved. The melt flow rate region disclosed in the publication is considerably large, which seems to indicate that it was not clear what combination of density and intrinsic viscosity would result in best processability and best clarity. Comparative examples shall show that many materials satisfying the definitions of EP-A447035 are not appropriate for producing films with a good combination of optical and mechanical properties.
EP-A-605952 discloses a polymer composition comprising two different ethylene polymers, which have been obtained by using a catalyst comprising at least two different metallocene compounds. This type of catalyst is sometimes referred to as dual site catalyst. The publication discloses that the two ethylene polymers may be polymerized separately and blended in an extruder, or the polymers may be dissolved and then combined. The polymers may also be produced in a two stage polymerization. The composition can be used to prepare films.
The examples show that the compositions where a dual site catalyst was used to prepare the polymer components produced films with good optical properties, high impact strength and good processability (or mouldability). Comparative examples 1 and 2 further disclose that compositions where a single component catalyst was used to prepare the polymer components, produced films with inferior optical and mechanical properties and poor processability.
EP-A-881237 discloses a two-stage process to produce ethylene polymers, wherein a metallocene catalyst based on a tetrahydroindenyl compound was used in a two-stage polymerization process. The document further discloses that the density of the polymer may range from 900 to 970 kg/m3 and the high load melt index (MFR21) from 0.1 to 45000 g/10 min. The examples disclose that the polymer was produced in loop and CSTR reactors. The materials disclosed in the examples had a density between 938 and 955 kg/M3 and a melt index MFR2 between 0.18 and 1.2 g/10 min. The use of the polymer was not disclosed and no practical examples concerning the use of the polymer were given.
EP-A-691367 discloses a film extruded from an in-situ blend of ethylene polymers prepared using Ziegler-Natta catalysts. The publication states that the resulting resins have a high mechanical strength. It is also stated in the publication that the film has a good processability and a low blocking tendency. The optical properties or gel level are not referred to. It is, however, known in the art that films made of such blends tend to be hazy.
EP-A-691353 discloses a process for preparing an in-situ blend giving a low gel level film. The process comprises polymerizing ethylene (with comonomer) in a cascade of gas phase reactors using a Ziegler-Natta catalyst. The publication further discloses that the resulting material has a good processability in blown film line.
WO-A-9618662 discloses a process for producing both high density and linear low density film material. The process comprises a cascade of a loop and a gas phase reactor. In the process, also a prepolymerizer is included. The publication also states that metallocene catalysts may be used in the process. However, it does not reveal the purpose for using a metallocene catalyst nor the advantages of it.
Thus, as apparent from the above, the available materials for films give limited alternatives in terms of a balance between clarity and mechanical properties.
It is an object of the present invention to provide an improved process for producing suitable polyethylene materials for the production of films.
It is another object of the present invention to provide novel polymer compositions for film-making.
It is a further object of the present invention to eliminate the problems of the prior art and to provide novel polymer films.
These and other objects, together with the advantages thereof over known processes and products, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
The present invention is based on the provision of bimodal polyethylene compositions comprising
a first (low molecular weight) component with MFR2 at least 10 g/10 min and a density higher than the density of the composition,
at least one other component,
said composition having a melt flow rate in the range MFR2=0.1-5.0 g/10 min and a density of 905-960 kg/m3.
According to one embodiment, the present invention provides a bimodal polyethylene compositions comprising
a first (low molecular weight) component with MFR2 at least 10 g/10 min and a density higher than the density of the composition,
at least one other component,
said composition having a melt flow rate in the range MFR2=0.1-5.0 g/10 min and a density of 915-960 kg/m3.
The composition is further characterized by a shear thinning index (SHI) of 3-20, viscosity of 5000-25000 Pas and storage modulus Gxe2x80x25kPa of 800-2500 Pa. It can be used for manufacturing polyethyiene films. The films according to the invention exhibit excellent balance between optical and mechanical properties.
The composition for polyethylene films can be produced by polymerizing or copolymerizing ethylene in a reactor cascade formed by at least two reactors in the presence of a metallocene catalyst capable of producing a high molecular weight polymer in the absence of hydrogen. The problem in using metallocene catalysts in the production of bimodal polyethylene has been that either they have not been able to produce a high enough molecular weight necessary in this kind of a process or their activity is too low to ensure an economic operation of such a process. Especially difficult has been to activate hafnium metal containing metallocene catalysts on a carrier. Now it has surprisingly been found that the catalysts according to the present invention, which are described in detail later on, are able to fulfil all the objectives and thus are suitable for the production of bimodal polyethylene in a process involving heterogeneous catalysis.
More specifically, the process according to the present invention is mainly characterised by what is stated in the characterising part of claim 1.
The present film-making process is characterised by what is stated in the characterising part of claim 13.
The present polyethylene film is characterised by what is stated in the characterising part of claim 18.
Considerable advantages are obtained by means of the present invention. The present process enables the preparation of resins having good optical properties (high gloss), processability similar to existing Ziegler-Natta materials and superior to that of unimodal metallocene materials (higher shear thinning index) and mechanical properties which are comparable to existing commercial materials (puncture, elongation) by using a metallocene catalyst.
The novel composition for films may be used for producing both blown and cast films. It is, however, particularly suitable for film blowing, with the improved optical properties.
The particular advantage of applying the invention in high density film production is that a material with very good optical properties can be obtained. This has not been possible using conventional methods which produce hazy films. Still, as will later be shown in the examples, the mechanical properties are on a good level.
Medium density films having a density range between 930-940 kg/m3 and a good clarity can be produced using conventional methods also. However, the invention allows for the combination of the good optical properties with good mechanical properties and a good processability.
As in medium density area, also in the low density area films having good optical properties have been known. But again, the combination of good optical properties and good mechanical properties is a new feature.